1. Field of the Invention
The present invention relates to optical disc reproduction and more particularly, to a method for generating a land/groove switching signal from pits on a land/groove (POLG) type disc and an apparatus therefor.
2. Description of the Related Art
In general, a read only memory digital video disc (DVD-ROM) has pits on a planar surface as opposed to a groove formed along tracks of the disc. So, the DVD-ROM is called a non-groove type optical disc.
As the need for recording a large quantity of information on an optical disc increases, a high-density recording medium is required. Accordingly, a POLG type optical disc such as a random access memory digital video disc (DVD-RAM), in which pits are formed on lands and grooves of the disc, has been suggested.
In a disc such as a DVD-RAM, information is recorded in the form of sector units. Each sector is roughly divided into a header information region having physical identification data (PID) and a user data region. Accordingly, the header information or user data contained in a radio frequency (RF) signal is read by a pickup and processed separately.
FIGS. 1A and 1B show the recording pattern of the header information on a DVD-RAM disc. In detail, FIG. 1A shows the header information recorded in the first sector of the track and FIG. 1B shows the header information recorded in other sectors.
In FIGS. 1A and 1B, reference numeral 50 represents a header information region and reference numeral 52 represents a user data region. The header information region 50 is divided into a peak header region 50a and a bottom header region 50b. The user data region 52 is divided into land and groove regions. A land/groove switching signal, which indicates a switch from a land track to a groove track or from a groove track to a land track, can be generated according to the sequence of the peak header region 50a and the bottom header region 50b.
Because the phase of a tracking error signal changes by 180.degree. when switching from a land to a groove, a disc reproduction apparatus selects the polarity (phase) of the tracking error signal according to the land/groove switching signal. Also, wobbles exist in the user data region 52. In FIGS. 1A and 1B, the wobbles represent wave-like sidewalls of the tracks (lands and grooves), formed between each land and groove. The wobble signal has information about the reference clock signal for controlling the spindle motor and about a channel clock signal for data playback.
FIG. 2 is a block diagram of a conventional land/groove switching signal generating apparatus.
The apparatus of FIG. 2 includes a pickup (P/U) 602, a radio frequency amplifier (AMP) 603, an offset compensator 604, an eight-to-fourteen modulation (EFM) comparator 606, an EFM phase-locked loop (EFM PLL) 607, a header envelope extractor 605, an amplitude modulation (AM) detector & header region information extractor 608, a phase comparator 609 and a tracking servo 610.
The apparatus shown in FIG. 2 operates as follows. An RF signal, output via a disc 601, the P/U 602 and the RF AMP 603, is provided to the offset compensator 604. The offset compensator 604 removes offset based on the central point of the EFM signal, regardless of whether the signal is from the header information region or the user data region, to output an EFM signal from which the offset has been removed. The EFM signal output from the offset compensator 604 is input to the EFM comparator 606.
FIG. 3 is a detailed block diagram of the EFM comparator 606. The EFM comparator 606 includes a first comparator 70, a low-pass filter (LPF) 72, a differential amplifier 74 and a gain determiner 76.
The first comparator 70 compares the EFM signal EFM output from the offset compensator 604 with a slice level Vp which is determined based on a feedback EFM signal EFMS and outputs a binary EFM signal EFMS according to the result of the comparison. That is, if the EFM signal EFM output from the offset compensator 604 is equal to or greater than the slice level Vp, the EFMS signal is output as a logic "1" value. Otherwise, the EFM signal is output as a logic "0" value.
The LPF 72 low-pass filters the feedback signal EFMS output from the first comparator 70 to obtain the average level thereof. Here, the LPF 72 has filtering characteristics corresponding to multiple speeds of the disc 601. This is because the amplitude and frequency of the EFM signal change according to the reproduction speed of the disc 601.
The differential amplifier 74 outputs the slice level Vp amplified from the difference between the output of the LPF 72 and a predetermined reference voltage Vref to the negative input terminal of the first comparator 70 and the gain determiner 76. Here, the predetermined reference voltage Vref represents a slice level when there is no offset. The gain determiner 76 determines the gain of the differential amplifier 74 in proportion to the slice level Vp.
The EFMS signal output from the EFM comparator 606 is input to the EFM PLL 607.
The EFM PLL 607 outputs a channel clock signal PCLK, whose phase is locked by the EFMS signal and data EFML reproduced by the channel clock signal PCLK, to the AM detector & header region information extractor 608. Here, the channel clock PCLK, a channel clock signal of the DVD, has a frequency of 29.16 MHz.
Further, the header envelope extractor 605 extracts a head peak signal HEADPK, indicating a peak header region, and a head bottom signal HEADBT, indicating a bottom header region from the RF signal output from the RF AMP 603, and outputs the extracted signals to the AM detector & header region information extractor 608. The envelope of the peak header has a peak value higher than the average peak level of the RF signal and the envelope of the bottom header has a peak value lower than that of the average peak level of the RF signal so that the signals HEADPK and HEADBT are generated based on the above. Here, rising edges of the signals HEADPK and HEADBT nearly match with the actual rising edges of the peak header and the bottom header. However, falling edges thereof are delayed from the actual falling edges of the peak header and the bottom header. This is because the signals HEADPK and HEADBT are usually generated by an integration method.
The AM detector & header region information extractor 608 extracts a signal HDPK, accurately indicating a peak header region, and a signal HDBT, accurately indicating a bottom header region, from the signals PCLK and EFML output from the EFM PLL 607 and the signals HEADPK and HEADBT, output from the header envelope extractor 605, to output to the phase comparator 609.
The phase comparator 609 compares the phases of the signals HDPK and HDBT and outputs a land/groove switching signal LGSEL indicating the positions of lands and grooves. The land/groove switching signal indicates the position of the track as a groove when the phase of HDPK leads the phase of HDBT. Meanwhile, when the phase of HDBT leads that of HDPK, the land/groove switching signal indicates the position of the track as a land.
The tracking servo 610 performs tracking control while switching the tracking polarity according to the state of the land/groove switching signal. The tracking servo 610 switches the polarity of the tracking error signal according to the land/groove switching signal, hence, accuracy of the land/groove switching signal LGSEL is very important for tracking servo control.
However, the header envelope extractor 605 may output the signals HEADPK and HEADBT in a region outside the actual header region due to unstable tracking control or a disc defect. In this case, the phase comparator 609 of the conventional land/groove switching signal generating apparatus shown in FIG. 2 may generate an incorrect land/groove switching signal based on incorrect HEADPK and HEADBT signals, causing a malfunction of the tracking servo. Accordingly, performance of the overall system becomes unstable.